The theme is improved understanding of cardiac electrophysiology and its application to clinical medicine. Methods employed are theoretic models, animal experimentation and patient observations. One portion of the program has the aim of developing an improved ECG examination for clinical use. This is being approached with large electrode arrays and computer processing to yield isopotential and other displays of potential patterns. Experimental and clinical projects are included. Another portion concerns the electrophysiologic state of ischemic tissue with relation to arrhythmias. Studies of the time course of changes in activation and recovery sequence and of fibrillation threshold in early ischemia are being conducted. ECG methods for estimating lesion size are being developed and will be applied to the determination of effects of arrhythmias on size. A third portion of the program includes multiple projects concerned with arrhythmias on a more general level. Studies of the relation of excitability to arrhythmias, computer modeling, effects of arrhythmias on coronary flow, patterns of ventricular responses to atrial fibrillation and others comprise this part of the program. In another portion of the program, electromechanical correlations including the effect of activation sequence and rhythm on cardiac output are under investigation. Autonomic influences on cardiac electrophysiology, electrocardiographic waveform and rhythm are being investigated. Clinical applications of findings in this area are being explored in the prolonged QT interval syndrome, neurological disease and in relation to sudden death. Another portion of the program concerns cardiac activation sequence with more extensive sampling than has been previously employed. Activation sequence in various portions of the heart under a variety of conditions including ischemic lesions and during arrhythmias is being determined.